Understanding development of the specialized ventricular cardiac conduction system is critical to generating biologic treatments for arrhythmia and mastering cardiac tissue regeneration. Arrhythmia and sudden death characterize several cardiomyopathies with aberrant trabeculation or noncompaction. The development and maintenance of trabeculated ventricular myocardium is a key process in heart organogenesis and appears linked to the normal development of a mature ventricular cardiac conduction pattern by way of erbb2/neuregulin (nrg) signaling. The erbb2/nrg pathway is already known to play a part in conduction system development. However, neither the process of trabeculation nor its role in setting the stage for the developing conduction system is understood. The long-term goals of this research are to describe the early structure and function of the specialized conduction system, and to determine how trabeculation relates to conduction system development. The specific aims of this project are twofold. First, we will establish molecular markers of the specialized ventricular cardiac conduction system in zebrafish. We will test the hypotheses that the location of the first cells of the conduction system are located in the trabeculating myocardial layer, and that trabeculation mutants lack conduction system cells as assessed by analyzing the expression of these markers. Second, we will test the hypothesis that the process of trabeculation, and not just the nrg/erbb2 pathway, is required for conduction system development. The zebrafish is a useful model organism for these experiments due to its genetic tractability, translucent embryogenesis, and similarity to human electrophysiological phenotypes and disease. To identify conduction cells, we will use in situ hybridization and antibody staining of candidate markers on whole-mount and sectioned zebrafish hearts. To assess conduction system development in wild-type, mutant, and genetically altered hearts, we will use optical mapping and embryonic ECG monitoring. We will assess conduction phenotypes in erbb2 mutants, as well as in wild-type embryos injected with heart-specific dominant negative constructs designed to block other pathways involved in trabeculation. Studying the structure and function of the developing cardiac conduction system and its relationship to trabeculation will further our understanding of arrhythmia in both congenital and acquired cardiomyopathies and will provide tools and knowledge that can help treat ventricular arrhythmia and sudden death by manipulating conduction cells.